As we get the same questions over and over again I thought it might be time to create a baseband dedicated thread.

If this thread helped you please press thanks, rate this thread with 5 stars and consider to buy me a beer!

The second post contains the FAQ. I will constantly expand this section, if you've a problem please check here before you are asking a question!
The FAQ currently lists answers to the following questions:

What is a Baseband, what is a RIL?

Which baseband is the best?

How do I install a RIL?

What RIL should I use? Should I install a matching RIL?

Wait, what is SU660?

Help, my baseband is shown as Unknown Baseband?

How can I check which baseband I have currently installed?

Why do I loose my RIL after flashing a ROM update?

Why is my call-quality that bad? Why is the person on the other end constantly hearing a disturbing static noise?

Here are all basebands I currently know of with MD5 checksums from a highspeed download mirror.
If there's something missing please post in the thread - I do check it regularly (but don't answer on questions which are covered by the FAQ or belong to Q&A).

I have now re-organized the Baseband Downloads and sort them by their type.
Note that you can use each Baseband for every ROM out there, you don't need to use a GB BB for a GB ROM.
This doesn't apply to RILs, see the FAQ to know which RIL to use on which ROM.
I have highlighted the recommended BBs from every section in blue.

I will constantly update it with frequently asked questions. If you miss anything just post your suggestion in this thread!

These are the questions:

What is a Baseband, what is a RIL?

Which baseband is the best?

How do I install a RIL?

What RIL should I use? Should I install a matching RIL?

Wait, what is SU660?

Help, my baseband is shown as Unknown Baseband?

How can I check which baseband I have currently installed?

Why do I loose my RIL after flashing a ROM update?

Why is my call-quality that bad? Why is the person on the other end constantly hearing a disturbing static noise?

Why do I have a timeoffset after installing a new Baseband?

What does "(V10b, Europe)" etc. in GetRIL mean?

Why isn't data coming back after turning off wifi?

What's the latest ROM/BB for my phone?

Help, I think I lost my IMEI?

What is a Baseband, what is a RIL?
A "rough" explanation: The baseband is the firmware for your hardware - the RIL is the driver for the ROM to communicate with the baseband. As each RIL is written for a specific baseband it is recommended that you use a matching BB/RIL combination.

Which baseband is the best?
The classic question.
First of all: As long as you don't have any problems you should stick to your baseband. Why bother changing it if everything is working fine?
You should only try to change the Baseband if you have problems like bad reception, bad in-call sound, no signal etc.

So.. which baseband is the best?
Unfortunately there is no good answer for this question as this depends on your phones specific hardware, your carrier, your reception, your network, the universe...
You have to try for yourself which one works best at your device! You should focus on the recommended Basebands from the first post - the other ones have no confirmed advantages and are listed for completeness.
While testing keep an eye on in-call quality, wifi and 2g+3g connectivity. Don't forget to install a proper RIL (see the 4th question).
Why I can't be more precise? Well... you have to see that there are some people who have only one BB with which their device will work properly - and we weren't able to find a pattern, yet.

But as I still get many PMs about my recommendations here you go:

Personally I would recommend the 725 BB from v10d with a matching RIL (see below), because it's considered stable and works for most people. 521 seems to be a good one, too.

If you're on the official ICS Stock ROM by LG I'd recommend an ICS BB.

Especially if you have in-call problems with P990 basebands (like me) - e.g. a bad voice clarity or background noise, you should try a SU660 BB.

I don't recommend SU660 Basebands if a P990 one is working for you! It might introduce some problems, see this post for some examples.
If you aren't happy with yours it's worth a try though - nothing can happen (if you backup).

How do I install a RIL?
Use the GetRIL app by sibbor or the LG RIL Version app by TrymHansen.
It's the easiest way, trust me. They will even create an entry in the custom_backup_list.txt file which will prevent the RIL from getting overwritten while updating your ROM.

If you want to install a RIL manually then download the "lge-ril.so" file and place it under "/system/lib"

What RIL should I use? Should I install a matching RIL?Attention: Do not mess with the RIL when you're using a CM10 or CM10-based ROM!
While it might be possible to change the RIL if you know exactly what you're doing, I don't accept any kind of discussion about it in this thread, as it will lead to problems for 99% of the people.
No one has confirmed any advantage from another RIL - so just don't touch it. You have been warned.

For LG ICS Stock ROMs you can change the RIL as long as you use another ICS RIL- but I'd still recommend to stick to the default RIL.
You have to be careful to not choose a "hacked" one from CM threads.

For older ROMs (Stock Froyo/GB, CM7, CM9, Hackfest CM10) you should install a matching RIL. While other RILs might work for you, there's no reason to not use the matching RIL for a specific baseband.
For LG Stock GB ROMs you need to install a RIL designed for GB.
For CM and Froyo ROMs use a RIL designed for Froyo or the SU660.
For BB725 you need to choose a v10x RIL for CM-based ROMs and a v20x RIL for Stock ROMs.

Attention: You cannot use a matching RIL while using a CM7 / CM9 / CM10 Hackfest ROM for the 313, 327 and 521 BBs because there doesn't exists a matching v10x RIL.
725 v10d, 420 and 218 RILs do work fine - but people have different experiences with in-call quality - check which one has the best quality with your phone.

Some more detailed information: (by TrymHansen)
When LG updated the P990 to Gingerbread, they decided to update how the RIL works internally. aremcee called it something like different "service codes".
With CM7/CM9 or Froyo ROMs you need to use RILs designed for the P990 Froyo ROMs or for the SU660. P990 Froyo Roms got the versions V10a-V10e and the SU660 has the versions V20f-V20j.
Stock Gingerbread ROMs HAVE TO use a RIL designed for P990 GB. This means any version from v20a to v20q (except v21x which is Canadian Froyo and 20f-20j which are for the SU660).

This graphic shows which RIL types work on which ROMs.

Wait, what is the SU660?
It's the Korean version of the LG Optimus 2x. It's similar but not identical to the P990, which means you shouldn't just use Software which is written for the SU660 on your P990.
Same thing applies for P999 aka T-Mobile G2x, the american version.

Help, my baseband is shown as Unknown Baseband?
If you're on the official LG ICS ROM this just means that you are not using an ICS Baseband.
Older ones will still work, don't worry - they just don't get displayed here anymore.
If it does annoy you, you can update the BB. But don't try to fix it by messing with the RIL!

If you're on a Froyo, GB, CM9 or Hackfest CM10 ROM it's an indication that you're using a wrong RIL for your Baseband.
For LG Stock GB ROMs you need to install a RIL designed for GB and for CM and Froyo ROMs use a RIL designed for Froyo or the SU660.
Please refer to the question "What RIL should I use? Should I install a matching RIL?" for more information.

How can I check which baseband I have currently installed?
Go to settings - About phone - Baseband version. The last four digits are your baseband version.
Note: Thanks to arcee at CM10 ROMs it now shows the exact baseband, not just it's date.
If this shows you an "Unknown Baseband" please refer to the above question.

Why do I loose my RIL after flashing a ROM update?
Each ROM comes with a RIL file which will overwrite your old one.

Why is my call-quality that bad? Why is the person on the other end constantly hearing a disturbing static noise?
You should try 824, 1120 or 420 BB with matching RIL. You probably have a "Made in Korea" P990 (see the sticker under the battery) which sometimes needs a SU660 Baseband to have a good voice clarity on CM-ROMs.
See here for more information.

Why do I have a timeoffset after installing a new Baseband?
You're using automatic time and installed a SU660 baseband which expects NITZ timestamps instead of GMT ones.
There are two solutions:
1.) Deactivate automatic time in Settings - Date & Time and use ClockSync to sync your time.
2.) If you're on a CM ROM other than the official CM7 nightly there's probably a NITZ workaround existing which gets controlled via the build.prop / local.prop.
Consult the thread of you ROM for more info.

What does "(V10b, Europe)" etc. in GetRIL mean?
The Europe flag in getRIL doesn't tell you where this RIL is best used but where it's from. "(V10b, Europe)" means that the RIL got extracted from the Europe version of the V10b LG Stock ROM.
Btw: I don't think that your country is importand for the BB decision even if people tend to ask if a BB works in their country.

Why isn't data coming back after turning off wifi?
That's an issue a couple users have with all ROMs and all Basebands.
It seems that there has been found a possible fix by dcseek.
Apperantly (at least for some users) it's caused by an IMEI loss (see the next question).
Follow the instructions in this post here to read more about the solution.
Or use this step-by-step by x_justin which included screenshots.

What's the latest ROM/BB for my phone?
You can do a reverse IMEI check. Add your IMEI at the end of the following link (dial *#06# in dialer to display it):

This is your latest official Stock ROM. If you want to extract it's BB you can do so with this tool: [TOOL] LGExtract 0.2.1.

Help, I think I lost my IMEI?
This does happen to some phones occasionally, especially when flashing a SU660 Baseband.
Type *#06# into the dialer to check if your IMEI is shown as "0" or as another number than the IMEI on the sticker of your box.
If it does please refer to this post to get instructions on how to restore your IMEI.

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Last edited by tonyp; 20th May 2013 at 01:47 AM.
Reason: Added "What's the latest ROM/BB for my phone?"

Download the newest drivers from here(if you don't already have the LG drivers package installed). If these drivers won't work uninstall your drivers and install these (older) ones!

Download a baseband file from the first post

Run the SmartFlashTool_P990.exe application on your PC

Turn device off, remove battery, plug device in while holding volume down - you should see a software update message on screen and the COM port should be filled automatically in the flash application below after installing the drivers if required. (Note: this might work without removing the battery, that depends on the phone!)

Now it depends on the Baseband you're trying to flash - if it's only one fls file or if there are four.

If there's only one fls file (for example: LGP990AT-00-V20s-EUR-XX-MAY-21-2012+0_CP.fls):

Select the 'ROM Copy D/L' tab

Ensure 'Normal Mode' is selected

Ensure 'Erase CAL' is NOT selected

Click the browse button next to 'CP Bin:' and select the baseband (FLS) file

Click 'Start' and wait for the flash to complete

If there are four fls files (R375_SIC.fls, AENEAS_FW.fls, slb.fls, psi.fls):

Use the 'Normal D/L' tab

Ensure none of the three "Erase.." checkmarks are set

Click on Add

Select all four files: R375_SIC.fls, AENEAS_FW.fls, slb.fls, psi.fls

Click 'Start' and wait for the flash to complete

Note: You could also flash a four fls file baseband from the 'ROM Copy D/L' tab by flashing all four files one by one (flash from big to small). But the above method is much easier.

Hint:
Additionally you can use the great All-In-One P990-Toolkit by spyrosk to flash your Baseband.
It will also help you with lots of other stuff, go check it out!

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